The Police have advised the Green Party that its investigation into whether the Government Communications Security Bureau's surveillance of Kim Dotcom was legal has been held up by the bureau becoming a party to Mr Dotcom's legal proceedings in the High Court.

The Police wrote to Green Co-leader Dr Russel Norman today to say that they expect to give him an update on the investigation early next year.

The Police said information and disclosure related to the court cases were impacting on the technical processes in the police investigation.

Police are investigating after Dr Norman asked them to establish whether the bureau breached the Crimes Act through its surveillance of Mr Dotcom.

"While we understand the difficulties involved in the court case proceeding while aspects of the GCSB's involvement are under investigation by the police, this investigation must come to a conclusion in its own right," Green Party Co-leader Metiria Turei said.

"Our spies are subject to the laws of this land. They must be held accountable by the Police and the Courts when they violate those laws.

"The Police, being the ones who called in the GCSB and who are now in court with the GCSB against Dotcom, must take extra effort to ensure the independence of their investigation into whether GCSB's illegal spying broke the Crimes Act.

"I take this letter to mean that Police are taking such issues seriously, and are formulating measures to make sure the public can have confidence in the robustness of their criminal investigation," Mrs Turei said.